organophosphates (or-ga-no-fos′fats)
A series of phosphorus-containing organic compounds usually also containing a halide ion that reacts with cholinesterase. O. phosphorylate cholinesterase and thus irreversibly inhibit it. Used as insecticides; have also been used as war gases.

organosol (or-gan′o-sol)
A hydrosol with an organic liquid instead of water as the dispersion means.

organ-specific
1. Denoting or pertaining to a serum produced by the injection of the cells of a certain organ or tissue that, when injected into another animal, destroys the cells of the corresponding organ. 2. Denoting an antigen specific for a particular organ.

orientation (or-e-en-ta′shun)
1. The recognition of one's temporal, spatial, and personal relationships and environment. 2. The relative position of an atom with respect to one to which it is connected, i.e., the direction of the bond connecting them. [Fr. orienter, to set toward the East, therefore in a definite position]
sexual o. concept that includes the permutations among body morphology, gender identity, gender role, and sexual preference.

Orientia (or-e-en′-a)
A member of the bacterial family Rickettsiae.
O. tsutsugamushi the only member of its genus, this species is the causative agent of scrub typhus, transmitted by mites; formerly called Rickettsia tsutsugamushi.

orientomycin (or′e-en-to-mi′sin)
SYN: cycloserine.

orifice (or′i-fis) [TA]
Any aperture or opening. SEE ALSO: aperture, opening, os, ostium, meatus. SYN: orificium [TA] . [L. orificium]
anal o. SYN: anus.
aortic o. [TA] the opening from the left ventricle into the ascending aorta; it is guarded by the aortic valve. SYN: ostium aortae [TA] , aortic ostium.
cardiac o. SYN: cardial o..
cardial o. [TA] the trumpet-shaped opening of the esophagus into the stomach. SYN: ostium cardiacum [TA] , cardiac opening, cardiac o., esophagogastric o..
esophagogastric o. SYN: cardial o..
o. of external acoustic meatus SYN: external acoustic pore.
external urethral o. [TA] 1. the slitlike opening of the urethra in the glans penis; 2. the external o. of the urethra (in the female) in the vestibule, usually upon a slight elevation, the papilla urethrae. SYN: ostium urethrae externum [TA] , external urinary meatus&star, external opening of urethra, meatus urinarius, orificium urethrae externum.
filling internal urethral o. [TA] the internal urethral o. once the bladder begins to be distended with urine, when the trigonal muscles are contracted and the detrusor muscle is relaxed; during this stage the o. occurs at a higher level, and is bounded by a different portion of the mucosa, than during voiding. SEE ALSO: voiding internal urethral o.. SYN: ostium urethrae internum accipiens [TA] .
gastroduodenal o. SYN: pyloric o..
golf-hole ureteral o. a circular and often escessively lateral ureteral o. that may be associated with vesicoureteral reflux, previous bladder surgery, or tuberculosis.
ileal o. [TA] the opening of the terminal ileum into the large intestine at the transition between the cecum and the ascending colon. SYN: ostium ileale [TA] , o. of ileal papilla&star, ileocecal opening, ileocecal o., ostium ileocecale.
o. of ileal papilla ileal o..
ileocecal o. SYN: ileal o..
o. of inferior vena cava SYN: opening of inferior vena cava.
o. of internal acoustic meatus SYN: internal acoustic pore.
internal urethral o. [TA] the internal opening or o. of the urethra, at the anterior and inferior angle of the trigone. SYN: ostium urethrae internum [TA] , internal urethral opening&star.
left atrioventricular o. [TA] an atrioventricular opening that leads from the left atrium into the left ventricle of the heart. SYN: ostium atrioventriculare sinistrum [TA] , mitral o., ostium arteriosum.
mitral o. SYN: left atrioventricular o..
pulmonary o. SYN: opening of pulmonary trunk.
pyloric o. [TA] the opening between the stomach and the superior part of the duodenum. SYN: ostium pyloricum [TA] , gastroduodenal o..
right atrioventricular o. [TA] an atrioventricular opening that leads from the right atrium into the right ventricle of the heart. SYN: ostium atrioventriculare dextrum [TA] , ostium venosum cordis, tricuspid o..
root canal o. an opening in the pulp chamber leading to the root canal.
o. of superior vena cava SYN: opening of superior vena cava.
tricuspid o. SYN: right atrioventricular o..
ureteric o. [TA] the opening of the ureter in the bladder, situated one at each lateral angle of the trigone; wide gaping of the ostium usually indicates vesicoureteral reflux. SYN: ostium ureteris [TA] , orificium ureteris, ureteral meatus, ureteral opening.
o. of uterus SYN: external os of uterus.
vaginal o. [TA] the narrowest portion of the canal, in the floor of the vestibule posterior to the urethral o.. SYN: ostium vaginae [TA] , orificium vaginae, vaginal opening.
o. of vermiform appendix [TA] the opening of the vermiform appendix into the lumen of the cecum. SYN: ostium appendicis vermiformis [TA] , ostium of vermiform appendix.
voiding internal urethral o. [TA] the internal urethral o. when the bladder is being emptied of urine, when the trigonal muscles are relaxed and the detrusor muscle is contracting; during this stage the o. occurs at a lower level, and is bounded by a different portion of the mucosa, than during filling and reserving. SEE ALSO: filling internal urethral o.. SYN: ostium urethrae internum evacuans [TA] .

origanum oil (o-rig′a-num)
The volatile oil (which contains carvacrol) obtained from various species of Origanum (family Labiatae); used as a rubefacient, as a constituent in veterinary liniments, and in microscopic techniques.

origin (or′i-jin)
1. The less movable of the two points of attachment of a muscle, that which is attached to the more fixed part of the skeleton. 2. The starting point of a cranial or spinal nerve. The former have two origins: the ental o., deep o., or real o., the cell group in the brain or medulla, whence the fibers of the nerve begin, and the ectal o., superficial o., or apparent o., the point where the nerve emerges from the brain. [L. origo, source, beginning, fr. orior, to rise]
o. of replication a sequence of the bacterial genome required for the initiating of a replicating fork by leading strand synthesis.

orizaba jalap root (o-riz′a-ba ja′lap)
SYN: ipomea.

Ormond
John K., U.S. urologist, *1886. See O. disease.

Orn
Symbol for ornithine or its radical.

ornate (or′nat)
A term that refers to the patterning of the scutum (gray or white markings on a dark background) in ixodid ticks. [L. ornatus, decorated]

Ornish
Dean, U.S. physician, *1953. See O. reversal diet.

ornithine (Orn) (or′ni-then, -thin)
2,5-Diaminovaleric acid;the l-isomer is the amino acid formed when l-arginine is hydrolyzed by arginase; not a constituent of proteins, but an important intermediate in the urea cycle; elevated levels seen in certain defects of the urea cycle.
o. acetyltransferase SYN: glutamate acetyltransferase.
o. δ-aminotransferase an enzyme that will reversibly catalyze the reaction of α-ketoglutarate and l-o. to form l-glutamate and l-glutamate γ-semialdehyde; a deficiency of this enzyme will result in gyrate atrophy of the choroid and retina. SYN: o. transaminase.
o. carbamoyltransferase an enzyme catalyzing formation of l-citrulline and orthophosphate from l-o. and carbamoyl phosphate; a part of the urea cycle; a deficiency of this enzyme will result in ammonia intoxication and impaired urea formation. SYN: o. transcarbamoylase.
o. decarboxylase an enzyme catalyzing the decarboxylation of l-o. to putrescine and CO2; first step in polyamine biosynthesis.
o. transaminase SYN: o. δ-aminotransferase.
o. transcarbamoylase SYN: o. carbamoyltransferase.

ornithinuria (or′ni-thi-noo′re-a)
Excretion of excessive amounts of ornithine in the urine.

Ornithodoros (or-ni-thod′o-rus)
A genus of soft ticks (family Argasidae), several species of which are vectors of pathogens of various relapsing fevers. They are characterized by a capitulum hidden below the hood and by disks and mamillae of the integument that are continuous from dorsal to ventral surfaces in a variety of patterns. [G. ornis (ornith-), bird, + doros, a leather bag]
O. coriaceus a tick species common in the mountainous coastal areas of California; adults readily attack deer, cattle, and humans, and have an irritating, painful, sometimes toxic bite. Transmits epizootic bovine abortion to cattle. SYN: pajaroello.
O. erraticus a species of tick the small variety of which is the vector of Borrelia crocidurae in Africa, the Near East, and Central Asia; the large variety is the vector of B. hispanica in the Spanish peninsula and adjacent north Africa.
O. hermsi a tick species that is a rodent parasite and vector of relapsing fever spirochetes, such as Borrelia hermsii, in the western U.S. and Canada.
O. lahorensis a species of tick that may transmit Borrelia persica, the agent of Persian relapsing fever.
O. moubata complex a group of four tick species in Africa; the taxonomy and ecology of this complex is of great significance because its members are vectors of relapsing fever spirochetes; members of the complex include O. moubata (various hosts), O. compactus (tortoises), O. apertus (porcupines), and O. porcinus (warthogs); a domestic subspecies of O. porcinus, in turn, forms three strains that feed chiefly on humans, fowl, and swine.
O. pappilipes the “Persian bug,” a tick species found in Central Asia and the Near East that transmits Borrelia persica, the pathogen in Iran of Persian relapsing fever.
O. parkeri a tick species found in the western U.S. and a vector of Borrelia parkeri.
O. rudis a tick species that is an important vector of relapsing fever spirochetes in Central and South America; possibly another complex similar to the O. moubata complex.
O. savigni a tick species transmitting Borrelia, an agent of relapsing fever of eastern Africa, southern Egypt, Ethiopia, and southwestern Asia.
O. talajé a tick species found in Mexico and in Central and South America, where it feeds on wild rodents, domestic animals, and humans; it delivers a painful, irritating bite and is a vector of Borrelia mazzottii, a cause of relapsing fever.
O. tholozani a species of tick that transmits Borrelia persica, an agent of relapsing fever in the Middle East and central Asia.
O. turicata a species of tick that readily attacks humans and other animals in the southern portion of the U.S. and Mexico; it is a vector of Borrelia turicatae, an agent of relapsing fever; the bite is painful and irritating.
O. venezuelensis a tick species that is the vector of Borrelia venezuelensis, agent of relapsing fever in Colombia, Venezuela, and mountainous parts of South America.
O. verrucosus a tick species, the vector of Borrelia caucasica.

Ornithonyssus (or-ni-thon′i-sus)
A genus of bird and rodent mites; species include O. bacoti, the tropical rat mite, a possible vector of murine typhus and a cause of human dermatitis; O. bursa, the tropical fowl mite; and O. sylviarum, the northern fowl mite. [G. ornis (ornith-), bird, + nyssus, to prick]

orodigitofacial (or′o-dij′i-to-fa′shal)
Relating to the mouth, fingers, and face.

orofacial (or-o-fa′shal)
Relating to the mouth and face.

orolingual (or-o-ling′gwal)
Relating to the mouth and tongue.

oronasal (or-o-na′sal)
Relating to the mouth and nose.

oropharyngeal (or-o-fa-rin′je-al)
Relating to the oropharynx.

oropharynx (or′o-far′ingks) [TA]
The portion of the pharynx that lies posterior to the mouth; it is continuous above with the nasopharynx via the pharyngeal isthmus and below with the laryngopharynx. SYN: pars oralis pharyngis [TA] , oral part of pharynx, oral pharynx. [L. os (or-), mouth]

orotidinuria (o-rot′i-den-u′re-a)
Elevated levels of orotidine in the urine; has been observed in defects in and inhibition of orotidylic acid decarboxylase.

orotidylate (OMP) (o-rot-i-dil′at)
A salt or ester of orotidylic acid.

orotidylic acid (OMP) (o-rot-i-dil′ik)
Orotidine 5′-monophosphate;an intermediate in the biosynthesis of the pyrimidine nucleosides (cytidine and uridine) that are found in nucleic acids. SYN: orotidine 5′-monophosphate.
o. decarboxylase an enzyme that catalyzes the conversion of OMP to UMP and CO2; a defect or inhibition of this enzyme will result in orotic aciduria and orotidinuria; this enzyme is a part of pyrimidine biosynthesis. Cf.:uridylic acid synthase. SYN: OMP decarboxylase.
o. phosphorylase SYN: orotate phosphoribosyltransferase.
o. pyrophosphorylase SYN: orotate phosphoribosyltransferase.

orphan (or′fan)
See o. products, under product. [G. orphanos]

orphenadrine citrate (or-fen′a-dren)
An antihistaminic that also has the same action and use as orphenadrine hydrochloride.

orphenadrine hydrochloride
It reduces spasm of voluntary muscles, probably by action on the cerebral motor areas; used in the symptomatic treatment of paralysis agitans and drug-induced parkinsonism.